Enhance logging across client, server, and ARQ layers

[mmdrn]

Summary

This PR adds comprehensive, structured logging throughout the VPN system to improve operational observability and debugging.

Logger Enhancement

• Added SubLogger type with With(key, value) chaining for automatic context propagation (e.g., [Sess:3] [Str:42])
• Added Warnf to the ARQ Logger interface

Session Lifecycle Logging

• Log session accepted with ID, cookie, and compression mode (INFO)
• Log session init failures, server busy, query build errors (WARN/DEBUG)
• Log rejected session close notices on server side (DEBUG)

Stream Lifecycle Logging

• Log stream creation with stream/session IDs (INFO)
• Log stream cleanup (DEBUG) and bulk close with count (INFO)
• Log server-side stream creation (DEBUG)
• Elevated SOCKS connected to INFO, added SOCKS failure logging (WARN)

Silent Error Elimination

• DNS response write failures in dns_listener.go (WARN)
• SOCKS handshake errors and unknown versions (DEBUG)
• Dispatcher encode/build failures (DEBUG) and no-valid-connections (WARN)
• Session close query build failures (DEBUG)
• Server buildNoDataResponseLogged now actually logs the reason (DEBUG)

Periodic Stats Reporting

• Client: 30s ticker logging active/pending/draining streams, valid resolvers, and per-resolver sent/acked/loss%/RTT
• Server: 30s ticker logging active sessions and total streams
• Added Balancer.Stats() method for aggregate connection statistics

ARQ Logging

• Data retransmissions with sequence number (DEBUG)
• Max retransmissions exceeded (WARN)
• Inactivity timeout with idle duration (INFO)
• Control packet expiry and retransmissions (DEBUG)

Design Decisions

• No external dependencies added — extends the existing custom logger
• No per-packet logging in hot paths to avoid performance impact
• Keeps the project's emoji/color tag style
• SubLogger satisfies the ARQ Logger interface for seamless integration

Files Changed (16)

• internal/logger/logger.go — SubLogger type
• internal/logger/logger_test.go — 4 new tests
• internal/arq/arq.go — Warnf interface + 5 log points
• internal/arq/arq_test.go — testLogger updated
• internal/client/session.go — session lifecycle logs
• internal/client/stream_client.go — stream lifecycle logs
• internal/client/socks_manager.go — SOCKS result logs
• internal/client/async_runtime.go — periodic stats logger
• internal/client/balancer.go — Stats() method
• internal/client/dispatcher.go — error path logs
• internal/client/dns_listener.go — write error log
• internal/client/tcp_listener.go — unsupported protocol log
• internal/udpserver/server.go — server stats logger
• internal/udpserver/server_session.go — close rejection log
• internal/udpserver/server_utils.go — NoData reason log
• internal/udpserver/stream_server.go — stream creation log

Testing

• All existing tests pass
• 4 new logger tests added (prefix, chaining, level filtering, Enabled)
• go build ./... clean

[mmdrn]

@masterking32 Please review this PR.

[mmdrn]

@masterking32 Conflicts resolved.